1. Field of the Invention
The present invention relates to a method and apparatus for modulation of a laser diode, and, more particularly, to a method and apparatus for minimizing electromagnetic interference resulting from modulation of a laser diode.
2. Description of the Related Art
Monochrome and color electrophotographic printers using laser scanning unit imaging require accurate, high-bandwidth modulation of the laser diode to produce high resolution halftone and color images. The time per pixel is determined by process speed, addressability, and laser scan efficiency. Higher speed and higher addressability printers possess shorter pixel times and higher video frequency content. Sub-pixel modulation used for half-tone or multi-level imaging at each pixel results in an even higher bandwidth requirement. Here, modulation is binary, i.e., the laser is switched between an ON state and an OFF or standby state.
The video path from the raster-image-processor data source to a properly designed laser driver typically limits the quality of image reproduction when tradeoffs are made to prevent undesirable electromagnetic radio-frequency emissions and interference. These tradeoffs typically involve adding inductors and capacitors to reduce electromagnetic interference at the expense of limiting or distorting video signals. Driver and receiver rise and fall times, turn-on and turn-off delays, and drive current capability also limit bandwidth and signal fidelity. Cable length, impedance, termination impedance, and losses further limit bandwidth and signal fidelity.
Lexmark printers using laser-scanning-unit printheads are now available with process speeds as high as 34 pages (8.5xe2x80x3xc3x9711xe2x80x3) per minute at 600 dots per inch (dpi) addressability and 17 pages per minute at 1200 dpi. Time per pixel for these printers is approximately 33.5 nanoseconds at 600 dpi and 18.6 nanoseconds at 1200 dpi. Each 600-dpi pixel is subdivided into eight 4.23 nanosecond slices, and each 1200 dpi pixel is subdivided into four 4.65 nanosecond slices. Each imaged pixel is ON for typically 25.4 nanoseconds (six of eight slices) at 600 dpi and 9.3 nanoseconds (two of four slices) at 1200 dpi. For consistent reproduction of half-tone images and fine-feature graphics, a laser diode 10 (FIG. 1) must faithfully reproduce the binary video signal generated at a raster image processor 12. This involves faithful transmission of the video signal over a cable 14 from a driver module 16 of raster image processor 12 to a receiver module 18 remotely located on a laser driver 20 of a laser scanning unit 22. This may be a distance d of 4 inches to 30 inches or more.
It is known to transmit a single-ended (not differential) signal over a twisted pair or shielded coaxial cable 24 (FIG. 2). The binary video signal at driver 16 is attenuated to a fraction of a TTL signal swing (from typically 3.4 V peak-to-peak to 1.8 V peak-to-peak) and level-shifted to be centered around the threshold voltage at receiver module 18. This approach minimizes signal swing and consequently radiated electromagnetic signal amplitude. However, this approach also introduces sensitivity to receiver, threshold variation, which can result in unexpected changes in laser ON time.
Another method of transmitting data that retains signal fidelity and minimizes electromagnetic interference is the use of a paired differential driver 26 (FIG. 3) and receiver 28. Here, electromagnetic interference is minimized to the extent that equal and opposite polarity signals are present in video cable 30, leading to cancellation of radiated radio frequency energy. Signal amplitude can be reduced for lower electromagnetic emissions by using a differential receiver 28 with less sensitivity to receiver threshold variation as compared to the single-ended receiver 18. Frequency content in leach of two signal wires 32, 34 is that of the high-frequency binary data. Commercially available differential driver-receiver pairs 26, 28 designed for high speed data tend to produce large switching currents. Using these drivers, differences in driver switching times and differences in line impedance have resulted in more electromagnetic interference than expected.
Serial-to-parallel conversion of data prior to transmission, and parallel-to-serial conversion at laser driver 20, as shown in FIG. 4, offers another way of keeping data frequency low over a video cable 36 interconnecting a raster image processor 38 and a laser scanning unit 40. However, to properly recreate the serial binary sub-pixel modulated data, both a data clock and a high-frequency gating signal must be carried in lines 42 and 44 of cable 36. This may create a worse electromagnetic compatibility problem than transmitting the serial data, since the gating signal has the same sub-pixel frequency content as the serial data for every pixel time. Another problem is that additional wires add to cost.
What is needed in the art is a simple and low cost method of passing a high bandwidth video modulation signal from a raster image processor to a laser print head driver with reduced electromagnetic interference.
Thee present invention provides a two-wire on-off encode and exclusive-OR decode of binary video data to pass a high bandwidth modulated video signal from source to receiver with reduced electromagnetic interference.
The invention comprises, in one form thereof, a method of driving a laser device in a electrophotographic machine. A binary image data signal having a first switching rate is provided. A plurality of binary encoded data signals are transmitted over a transmission medium. Each of the encoded data signals is dependent upon the image data signal and has a respective second switching rate lower than the first switching rate. The image data signal is recreated by using the encoded data signals. The laser device is driven with the recreated image data signal.
The invention comprises, in another form thereof, an apparatus for driving a laser device in an electrophotographic machine. The apparatus includes a raster image processor having an image data source outputting a binary image data signal with a first switching rate. A data encoder receives the binary image data signal and transmits a plurality of binary encoded data signals over a transmission medium. Each of the encoded data signals is dependent upon the image data signal and has a respective second switching rate lower than the first switching rate. A laser scanning unit includes a data decoder receiving the binary encoded data signals over the transmission medium and recreating the image data signal by using the encoded data signals. A laser driver drives the laser device with the recreated image data signal.
An advantage of the present invention is that electromagnetic interference is reduced at low cost.
Another advantage is that signal quality is sufficient to reproduce the sub-pixel modulation required for halftone reproduction in a 20 pages per minute, 1200 dpi addressable electrophotographic printer using laser scanning unit imaging.